Signal mixers are widely used in radio transceivers for wireless applications including telemetry systems, wireless Local Area Networks, and communication devices such as radios, cellular telephones and other radio frequency (RF) devices. There are two types of mixer, a passive mixer and an active mixer. A mixer may be either passive or active according to the mixer's ability to provide gain or not. A passive mixer is based on an un-biased configuration (i.e. no gain) and it generally exhibits a poor noise figure but a desirably high linearity over a relatively wide dynamic frequency range. The use of negative feedback configurations to amplify and buffer weak signals from the passive mixer is commonly used in integrated circuit applications. However, in such architectures, the feedback is connected directly to the input stages of the operational amplifier and therefore has an impact on the mixer stages. In contrast, an active mixer, such as a Gilbert cell mixer, has good gain and noise figure. However, active mixers have relatively poor linearity. In order to increase the linearity, increased bias current is needed which leads to increased power consumption. This is not desirable in modern day devices where the devices are typically small in size (heating issue) and operate on rechargeable batteries (limited power).
In wireless applications, the transmitted and received signal is an RF signal. The RF signal consists of a baseband signal, which is a relatively low frequency signal, modulated on a relatively high frequency signal commonly known as a carrier frequency signal. Mixers are used in transceivers to convert a low frequency signal (e.g. baseband signal) to a high frequency signal or a high frequency signal (e.g. RF signal) to a low frequency signal by mixing the signal with a local oscillator signal. Therefore, the outputs from a mixer are the sum (fRF+fLO) or the difference (fRF−fLO or fLO−fRF) of the input frequencies signals. In the case of a downconversion, the output signals from the mixer are then passed through a filter to remove unwanted frequencies before sending the filtered signal (which is typically a baseband signal) to an information recovery module.
Individually, current passive and active mixers are unable to provide high linearity, low power consumption and good noise figures. As such, it is desirable to provide a signal downconverter that provides the linearity of a passive mixer, gain control and a good noise figure of an active mixer and low power consumption.